Method and device for pressure regulation of a well

ABSTRACT

A method for the pressure regulation of a well ( 1 ) extending into a well formation ( 2 ) and being drilled from a drilling vessel at sea, the well ( 1 ) including a blowout preventer ( 10 ) on the seabed ( 12 ) and a riser ( 14 ) extending from the blowout preventer ( 10 ) up to the vessel, and the return path ( 8, 14, 20 ) of a drilling fluid ( 24 ) being provided with a rotatable surface seal ( 22 ) sealingly surrounding a drill string ( 4 ), the rotatable surface seal ( 22 ) being arranged to seal against a pressure difference between the surroundings and the return path ( 8, 14, 20 ), and there being arranged, at the rotating surface seal ( 22 ), a closable drilling-fluid outlet ( 28 ), the method including: arranging a pump inlet ( 32 ) from the return path ( 8, 14, 20 ) under water; connecting a pump ( 30 ) to the return path ( 8, 14, 20 ), the pump ( 30 ) being connected to a pressure pipe ( 34 ) discharging on the vessel; monitoring the drilling-fluid pressure; and controlling the pumping rate through the pump ( 30 ) to maintain a substantially constant drilling-fluid pressure at the well formation ( 2 ), both when drilling fluid ( 24 ) is flowing and when it has been stopped.

This invention relates to a method for the pressure regulation of a well. More particularly, it relates to a method for the pressure regulation of a well extending into a well formation and being drilled from a vessel at sea, the well including a blowout preventer on the seabed and a riser extending from the blowout preventer up to the vessel, and the return path of the drilling fluid being provided with a rotating surface seal sealingly surrounding a drill string, the rotating surface seal being arranged to seal against a pressure difference between the surroundings and the return path, and there being arranged a closable drilling-fluid outlet at the rotating surface seal. The invention also includes a device for practising the method.

When drilling wells in the ground, it may happen that the pore pressure pp of a well formation differs relatively little from the fracture pressure fp.

Normally, the specific weight of the drilling fluid is adjusted to be such that the pressure head of the drilling-fluid column resists the pore pressure, preventing well fluid from entering the borehole during drilling.

However, drilling fluid must also be able to fill other functions. For example, it must be relatively viscous to carry cuttings up to the surface.

The drilling-fluid pressure in the formation is made up of, inter alia, the static pressure from the fluid column and a dynamic pressure, the so-called frictional pressure, from the flowing drilling fluid.

While the static pressure is relatively constant, the frictional pressure disappears when the drilling-fluid circulation is stopped, which is repeated every time a new pipe is to be connected to a drill string.

To counteract inflow of well fluid into the well during such operations, the static pressure from the drilling fluid must, as mentioned above, be sufficiently large to prevent inflow of well fluid from the well formation, but not so large that it exceeds the fracture pressure so that drilling fluid will flow into the well formation.

As the drilling-fluid circulation is restarted, the frictional pressure may, when there is little difference between the pore pressure and the fracture pressure, make the fracture pressure be exceeded. Drilling fluid may then flow into the formation. Apart from the loss of relatively expensive drilling fluid, such outflow, often termed “lost circulation”, may also entail damage to the formation, for example cause reduced production later on, and also considerable time lost in connection with the drilling operation.

When using conventional drilling techniques in which the drilling fluid has atmospheric pressure at the surface, it is not always possible to regulate the static pressure sufficiently to compensate for the frictional pressure when the circulation of drilling fluid is started and stopped. In some cases the dynamic frictional pressure is larger than the difference between the pore pressure and the fracture pressure.

It is known to place a choke in the return pipe for drilling fluid, normally at a level under water in the annulus between the drill string and a riser, and to use a pump to pump the drilling fluid past the restriction, see for example DeepVision's “Delta Vision”, see WO 2003/006778, and Statoil ASA's “Annular Pressure Control system”. The static pressure below the restriction and thus also in the formation may be changed relatively rapidly by controlling the pumping rate. The necessary flow restriction constitutes a complicating component in the drilling system.

It is also known to arrange a seal around the drilling string at the upper portion of the return pipe to be able to pressurize the drilling fluid. The seal must allow rotation of the drill string and is usually termed a Surface Rotating Control Device or just Rotating Control Device (RCD). The method is termed Pressurized Mud-Cap Drilling (PMCD). It is thus possible to pressurize the drilling fluid and regulate this pressure across a choke valve. When drilling fluid is not circulated, the pressure of the drilling fluid may rapidly be increased to maintain the pressure in the well. The applied pressure can rapidly be reduced when the drilling fluid is circulated again. The method has limitations and drawbacks in that the specific weight of the drilling fluid is no longer large enough to maintain the pressure in the well, unless the return path is pressurized.

The PMCD method is described in the article “MPD techniques address problems in drilling Southeast Asia's fractured carbonate structures” in Drilling Contractor which is the official magazine of The International Association of Drilling Contractors, pages 34-36, November/December 2006.

It may also happen that the pore pressure is so low that it cannot resist the pressure from a fluid column that extends to the surface, or that the difference between the pore pressure and the fracture pressure is so small that a pressure gradient must be constructed, based on fluids of different specific weights. NO patent 319213 discloses a method in which the return path is filled with a fluid which has a lower specific weight than the drilling fluid.

The invention has for its object to remedy or reduce at least one of the drawbacks of the prior art.

The object is achieved according to the invention through the features which are specified in the description below and in the claims that follow.

A method has been provided for the pressure regulation of a well extending into a well formation and being drilled from a vessel at sea, the well including a blowout preventer on the seabed and a riser extending from the blowout preventer up to the vessel, and the return path of the drilling fluid being provided with a rotating surface seal sealingly surrounding a drill string, the rotating surface seal being arranged to seal against a pressure difference between the surroundings and the return path, and there being arranged a closable drilling-fluid outlet at the rotating surface seal. The method is characterized by including:

-   -   arranging a pump inlet from the return path under water;     -   connecting a pump to the return path, the pump being connected         to a pipe discharging on the vessel;     -   monitoring the drilling-fluid pressure; and     -   regulating the pumping rate through the pump to maintain a         substantially constant drilling-fluid pressure at the well         formation, both when drilling fluid is flowing and when it has         been stopped.

As long as there is a sufficient difference between the drilling-fluid pressure in the well formation and the fracture pressure, drilling can be carried out in a manner known per se, by the closable drilling-fluid outlet being opened. Drilling fluid thereby flows back via the return path, which is formed, in this preferred embodiment, by inter alia an annulus between the riser and the drill string. Since chokes are unnecessary in the return path, the return drilling fluid may flow freely in the return path.

If there is inflow of drilling fluid into the formation, the closable drilling-fluid outlet may be closed in order thereby to be able to establish an overpressure or underpressure in the return pipe relative to the surroundings. By starting the pump, the static pressure in the well formation can be reduced immediately, for example by the drilling-fluid level being lowered in the return path, whereby an underpressure is established in the upper portion of the return path.

Cuttings and drilling fluid may thus be circulated out of the well, also when the fluid column in the return path is at a lower height level, so that drilling fluid can no longer flow out via the closable drilling-fluid outlet.

If the prevailing pressures in the formation are too low to resist the pressure from the pressure column of the drilling fluid, at least a portion of the return path that is above the pump inlet may be filled with a second fluid of a different specific weight, normally lower, in relation to the drilling fluid. The static pressure from the fluid column may thereby be adjusted to wells in which both the formation pressure and the fracture pressure are relatively low.

When a second fluid is in the return path, the pumping rate may be controlled also to keep the interface between the drilling fluid and the second fluid at an approximately constant height level.

The method may be practised by means of a pressure-regulation device for a well extending into a well formation and being drilled from a vessel at sea, the well including a blowout preventer on the seabed and a riser extending from the blowout preventer up to the vessel, and the drilling-fluid return path being provided with a rotating surface seal surrounding a drill string, the rotating surface seal being arranged to seal against a pressure difference between the surroundings and the return path, and there being arranged a closable drilling-fluid outlet at the rotating surface seal, and the pressure-regulation device being characterized by a pump inlet from the return path being arranged under water and a pump being connected to the return path, the pump being connected to a pipe which discharges on the vessel, the pumping is rate being adjustable to maintain a substantially constant drilling-fluid pressure in the well formation, both when drilling fluid is flowing and when it has been stopped.

At least a portion of the return path that is above the pump inlet may be filled with a second fluid which has a different specific weight in relation to the drilling fluid.

If the fluids in the return path show a tendency to mix, a physical partition, for example in the form of a piston-like component, may be arranged between the drilling fluid and the second fluid.

When the closable drilling-fluid outlet is open, the device corresponds to what is described in NO 319213.

The pumping rate may be controllable to be able to keep an interface between the drilling fluid and the second fluid at an approximately constant height level.

Known methods that include a rotatable surface seal are formed to utilize a static pressure from the drilling fluid at the formation, the static pressure being selected to be suitable when there is circulation of drilling fluid. The fluid column is subjected to extra pressure to prevent inflow of well fluid when the drilling-fluid circulation is stopped. This makes it difficult to open the system to replace a rotating seal and so on. In accordance with the method and the device according to the invention, the static pressure may be adjusted to the formation pressure of the well formation when there is no circulation of drilling fluid, and this static pressure may be reduced by means of the pump to avoid exceeding the fracture pressure as circulation is started. When the circulation has stopped, the system may then freely be opened as the drilling fluid itself maintains a sufficient static pressure in the formation, as in conventional drilling.

Thereby, it is possible to utilize a heavier drilling fluid than what is possible by means of the prior art, while, at the same time, cuttings and drilling fluid may be circulated out of the well.

The method and the device according to the invention also render a rotating seal in the riser or at the seabed superfluous.

Such seals must be pulled out every time the drill pipe is to be pulled out, which is both complicated and time-consuming.

In what follows is described an example of a preferred method and embodiment which is visualized in the accompanying drawings, in which:

FIG. 1 shows schematically a section of a well in accordance with the invention; and

FIG. 2 shows schematically a section of a well in accordance with the invention in an alternative embodiment.

In the drawings, the reference numeral 1 indicates a well which extends down into a well formation 2 and in which a drill string 4, which is provided with a drill bit 6, extends from a vessel not shown into the well formation 2. A casing 8 is connected to a blowout preventer 10 on the seabed 12.

A riser 14 enclosing the drill string 4 extends from the vessel, not shown, down through the sea surface 16 to the blowout preventer 10. The riser 14 is suspended from the vessel not shown by means of a riser tensioner 18, the telescopic pipe 20 of the riser 14 being provided with a rotatable surface seal 22. The rotatable surface seal 22 is arranged to seal against the drill string 4 also when the drill string 4 is rotating.

The casing 8, riser 14 and telescopic pipe 20 form a return path for a drilling fluid 24.

At a height level somewhat lower than that of the rotatable surface seal 22, the telescopic pipe 20 is provided with a drilling-fluid outlet 28 closable by means of a valve 26.

A pump 30 is connected to a pump inlet 32 on the riser 14 and to a pressure pipe 34 extending to the vessel not shown. A pressure gauge 36 communicates with the riser 14 at the pump inlet 32.

During normal drilling, drilling fluid may return via the return path 8, 14, 20 and flow out via the closable drilling-fluid outlet 28. If measurements should show that drilling fluid 24 is entering the well formation 2, the pump 30 may be started while, at the same time, the closable drilling-fluid outlet 28 is closed by means of the valve 26.

The static pressure from the drilling-fluid column is reduced by reducing the drilling-fluid column height, whereby the fracture pressure is no longer exceeded. When the circulation of drilling fluid 24 is stopped, the static pressure is increased by stopping or reducing the flow rate through the pump 30 to prevent the entrance of well fluid from the well formation 2.

The height position of the pump inlet 32 is determined from the prevailing pressure conditions in the well formation 2, the maximum pressure reduction achievable being the pressure head of the of the drilling-fluid column between the rotatable surface seal 22 and the pump inlet 32.

In an alternative method and embodiment, there is placed, in a portion of the riser 14, a second fluid 38 which has a different specific weight in relation to the drilling fluid 24. An interface 40 is established between the drilling fluid 24 and the second fluid 38, see FIG. 2.

If the valve 26 is open, the device functions in a manner corresponding to the method according to NO 319213 in which the advantages of a so-called dual-gradient system is described. The method according to NO 319213 does not include a rotatable surface seal 22.

In the same manner as described above and with the valve 26 closed, this alternative method makes it possible for a relatively rapid change of the static pressure in the drilling fluid to take place. 

1. A method for the pressure regulation of a well (1) extending into a well formation (2) and being drilled from a drilling vessel at sea, the well (1) including a blowout preventer (10) on the seabed (12) and a riser (14) extending from the blowout preventer (10) up to the vessel, and the return path (8, 14, 20) of a drilling fluid (24) being provided with a rotatable surface seal (22) sealingly surrounding a drill string (4), the rotatable surface seal (22) being arranged to seal against a pressure difference between the surroundings and the return path (8, 14, 20), and there being arranged, at the rotating surface seal (22), a closable drilling-fluid outlet (28), characterized in that the method includes: arranging a pump inlet (32) from the return path (8, 14, 20) under water; connecting a pump (30) to the return path (8, 14, 20), the pump (30) being connected to a pressure pipe (34) discharging on the vessel; monitoring the drilling-fluid pressure; and controlling the pumping rate through the pump (30) to maintain a substantially constant drilling-fluid pressure at the well formation (2) both when drilling fluid (24) is flowing and when it has been stopped.
 2. The method in accordance with claim 1, characterized in that the closable drilling-fluid outlet (28) is opened.
 3. The method in accordance with claim 1, characterized in that the closable drilling-fluid outlet (28) is closed, whereby overpressure or underpressure may be established in the return path (8, 14, 20) relative to the surroundings.
 4. The method in accordance with claim 1, characterized in that at least a portion of the return path (8, 14, 20) that is above the pump inlet (32) is filled with a second fluid (38) which has a different specific weight in relation to the drilling fluid (24).
 5. The method in accordance with claim 4, characterized in that the pumping rate through the pump (30) is controlled to keep an interface (40) between the drilling fluid (24) and the second fluid (38) at an approximately constant height level.
 6. A pressure-regulation device for a well (1) extending into a well formation (2) and being drilled from a vessel at sea, the well (1) including a blowout preventer (10) on the seabed (12) and a riser (14) extending from the blowout preventer (10) up to the vessel, and the return path (8, 14, 20) of the drilling fluid being provided with a rotating surface seal (22) sealingly surrounding a drill string (4), the rotating surface seal (22) being arranged to seal against a pressure difference between the surroundings and the return path (8, 14, 20), and there being arranged a closable drilling-fluid outlet (28) at the rotating surface seal (22), characterized in that a pump inlet (32) from the return path (8, 14, 20) is arranged under water and a pump is connected to the return path (8, 14, 20), the pump (30) being connected to a pressure pipe (34) which discharges on the vessel, the pumping rate being adjustable to maintain a substantially constant drilling-fluid pressure at the well formation (2), both when drilling fluid (24) is flowing and when it has been stopped.
 7. The pressure-regulation device in accordance with claim 6, characterized in that at least a portion of the return pipe (8, 14, 20) that is above the pump inlet (32) is filled with a second fluid (38) which has a different specific weight in relation to the drilling fluid (24).
 8. The pressure-regulation device in accordance with claim 7, characterized in that the pumping rate is controllable to keep an interface (40) between the drilling fluid (24) and the second fluid (38) at an approximately constant height level. 